The development of therapeutic agents capable of specifically targeting cancer cells and tumor-associated microenvironments including tumor blood vessels remains an important goal.
One strategy to achieve a high local concentration of chemotherapeutic drugs in tumor tissues is the incorporation of a targeting ligand able to actively guide the therapeutic agents to antigens or receptors uniquely expressed or over-expressed on the target cells relative to normal tissues. Various targeted drug delivery systems have been designed to contain antibodies or antibody Fab′ fragments, lectins, proteins or peptides as targeting ligands to direct chemotherapeutic drugs selectively to cancer cells. However, when constructing targeted drug delivery systems, issues of the degree of receptor expression, heterogeneity in receptor expression among different tumor cells, binding affinity of the ligands for their receptor and the occurrence of receptor-mediated internalization might limit the choice of targeting ligands that are available for active drug targeting.
Certain polycationic sequences [also termed cell-penetrating peptides (CPPs) or protein transduction domains (PTDs)] can bring covalently attached payloads into mammalian cells without requiring specific receptors. Such proteins or peptides contain domains of less than 20 amino acids that are highly rich in basic residues, and have been used for intracellular delivery of various cargoes with molecular weights significantly greater than their own. These peptides include the 60 amino acid Antennapedia (Antp) (from Drosophila), the penetratin homeodomain derived peptide sequence (RRMKWKK (SEQ ID NO: 1) the HIV-1 Tat protein (TATp), The VP22 protein (DAATATRGRSAASRPTERPRAPARSASRPRRPVD (SEQ ID NO: 2) from the Herpes Simplex Virus type-1, the chimeric peptides such as transportan (GWTLNSAGYLLKINLKALAALAKKIL (SEQ ID NO: 3)), and synthetic polyarginines, such as R9.
However, several biologic features limit CPP usefulness in living animals, most significantly being the lack of cell specificity. All the CPPs are highly positively charged, presenting basic residues of lysine or arginine. These cationic oligopeptides are able to attach rapidly and strongly to the cell surface through non-specific electrostatic interactions with the negative charges present of anionic phospholipids and glycosaminoglycans. In fact, upon administration (intravenous, IV, or intraperitoneal, IP), CPPs and their therapeutic conjugates are dispersed almost all over the body and can be found in blood cells, lung, liver, kidney and other tissues, even in the brain, indicating the penetration through the blood brain barrier (BBB). Therefore in drug delivery systems (DDS) the use of conventional CPPs was generally limited by the nature of the cargo molecule and its ability to keep healthy cells unharmed due to non-specific cell penetration.
While some efforts were made to enhance cell uptake specificity, including fusing the CPP to a cleavable linker, which prevents uptake unless cleavage occurs, such systems are not terribly versatile or reliable.
There remains a need for effective targeting of diseased cells and tissue and thereby effective diagnostic and therapeutic targeted delivery systems of high sensitivity.